The present invention relates to liquid level sensors in general and to sensors employing reed switches in particular.
Determining the amount or level of liquid in a tank is a problem of widespread importance. Applications range from fuel tanks, lubricating fluid tanks, and water level monitors within humidifiers, boilers, and dishwashers. Typically the concerns are the same: to reliably indicate when liquid rises to a preselected level or when a liquid falls below a preselected level. A common mechanical float sensor incorporates a buoyant float, which responds to a rise in the level of a liquid by pushing against a switch causing the switch to open or close.
Float sensors are typically a critical part of an apparatus, serving as part of the liquid control system in the apparatus. The flooding or running dry which results from the float sensor""s failure can be costly or at least inconvenient. Float sensors employing electric switches face additional challenges. An electric switch submerged in or positioned near a liquid has inherent reliability problems. The liquid may be corrosive, may promote galvanic corrosion, or may form a varnish, which builds up on exposed surfaces and contacts resulting in the buildup of an insulating layer, which affects electrical parts.
Alternatively, the raising float may contain a magnet, which interacts with a reed switch causing it to open or close. Reed switches are widely used where an extremely reliable switch is required. Reed switches are reliable because the contacts that close the switch are located within a hermetically sealed glass envelope. A reed switch typically has two ferromagnetic reeds that extend from opposite ends of a sealed glass tube. The contacts are formed on opposed surfaces of the reeds which overlap and are spaced apart a small amount when no magnetic field is present. In the presence of a magnetic field the ferromagnetic reeds attract and are brought into engagement at the contact surfaces, thus closing a circuit between the ferromagnetic reeds.
Reed switches do have some limitations which flow from their advantages: namely the glass capsule which contains the ferromagnetic reeds is inherently subject to being broken, and a reed switch must be reliably positioned with respect to the actuation magnet in a cost affective manner.
What is needed are float sensors employing reed switches which are more reliable and more easily installed.
The float sensor of this invention employs a reed switch contained within a solid plastic body which is positioned by a clip adjacent to an impervious wall forming part of a fluid container. Addition or drainage of the fluid causes the fluid level within the fluid container to vary, opposite the reed switch, this in turn causes a float to move toward or away from the reed switch. This motion results in turning the reed switch on or off.
The plastic body is formed by injecting glass filled polyester into a die that surrounds a reed switch with two high stiffness leads that form part of a lead frame. The reed switch is oriented within the die so that when plastic is injected, it flows along the glass tube of the reed switch. This flow path minimizes the disturbance of the reed switch due to the plastic flow that could result in the reed switch becoming broken. The molded plastic body incorporates an integrally formed clip arm that functions to position and hold the plastic body onto a larger structure.
The reed switch may be actuated by a magnet mounted to a float, which is pivotally mounted by an arm on a structure. The arm has the magnet mounted between the float and a pivot. Motion of the float in response to fluid level changes results in movement of the magnet into and out of position with respect to a reed switch contained within a plastic body, to cause the reed switch to open and close with the motion of the magnet. The reed switch is mounted spaced from the float by an impervious wall that prevents moisture or liquids from contracting the plastic body.
An alternative means for reed switch actuation consists of a torpedo shape float contained within a cylindrical tube. The bottom of the tube has openings through which fluid can enter to raise the float and cause a magnet contained in the float to move relative to a reed switch within a plastic body, causing the reed switch to open or close. The float is separated from the plastic body by an impervious wall.
A further alternative reed switch actuation means consists of a float containing a magnet, which is constrained to move along opposed rails which capture the float. The float is again separated from a plastic body containing a reed switch by an impervious wall. Motion of the float due to liquid level changes causes the reed switch to change the activation state.
It is a feature of the present invention to provide an improved packaging for a reed switch.
It is a further feature of the present invention to provide float sensors that are more reliable, more easily assembled, and low cost.
It is another feature of the present invention to provide a method of encapsulating a reed switch.
Further features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.